1. Field of the Invention
This invention relates to metallic oxide detectors for reducing gases.
2. Description of the Prior Art
The use of a thin film of metallic oxide, such as tungsten oxide, whose surface has been activated by the deposition of a platinum catalyst, to make a quantitative analysis of hydrogen or other reducing gases is described in U.S. Pat. No. 3,479,257, issued Nov. 18, 1969, to Paul J. Shaver. As distinguished from conventional catalytic combustion sensors, such as the sensor element described in U.S. Pat. No. 3,586,486, issued June 22, 1971 which utilize a change in element resistance due to a change in temperature to provide the concentration indication, the sensor in U.S. Pat. No. 3,479,257 initiates a chemical reduction reaction which changes the carrier density in the oxide film as a function of the gas reduced, thereby producing a change in film resistance to provide the concentration indication.
The sensors described in U.S. Pat. No. 3,479,257 may be utilized for the quantitative analysis of various gases such as hydrogen, hydrogen sulfide, and anhydrous ammonia, and hydrazine, propane, butane, methyl alcohol, and ethyl alcohol. Such sensors have no specific selectivity with respect to any one of these substances and so are unable to distinguish in mixtures of reducing gases, as to any single constituent. It has been found that in actual practice where hydrogen sulfide or ammonia are to be quantitatively analyzed by such devices, the ambient atmosphere will contain, as a background, significant quantities of other reducing gases to which the device is sensitive, typically hydrogen in concentrations much higher than the gas to be detected. Inherently, the sensor, which monitors the change in carrier density, does not discriminate between the various reduction reactions which occur.
Thus, the sensor is sensitive to reducible gas concentration in general, but not to any particular reducible gas component. Such a sensor is generally more sensitive to hydrogen than to other reducing gases. The presence in combination of hydrogen or other reducing gas with the hydrogen sulfide or ammonia to be analyzed will produce an erroneous concentration indication for the quantitative analysis. Therefore, for all practical purposes, the device manufactured in accordance with the teaching of the aforesaid patent is not satisfactory for the quantitative analysis of hydrogen sulfide or ammonia.
A variation of the type of detector of U.S. Pat. No. 3,429,257 which is particularly adapted to measure hydrogen sulfide concentration is described in U.S. Pat. No. 3,901,067, issued Aug. 26, 1975. In this patent, a device is described which uses a stannic oxide semiconductor film which is doped with selected metals to provide hydrogen sulfide sensitivity.